Closed loop temperature controlled circuit to improve device stability

ABSTRACT

An integrated circuit is provided having an active circuit. A heating element is adjacent to the active circuit and configured to heat the active circuit. A temperature sensor is also adjacent to the active circuit and configured to measure a temperature of the active circuit. A temperature controller is coupled to the active circuit and configured to receive a temperature signal from the temperature sensor. The temperature controller operates the heating element to heat the active circuit to maintain the temperature of the active circuit in a selected temperature range.

BACKGROUND

1. Technical Field

The present disclosure relates to integrated circuits. The presentdisclosure relates in particular to the field of temperature control ofan integrated circuit.

2. Description of the Related Art

Integrated circuits are used to perform many functions and are found innearly all electronic devices. Integrated circuits are typically formedwithin and on semiconductor substrates. The physical properties of thesemiconductor substrate affect the functionality of the integratedcircuit. The physical properties of the semiconductor substrate are inturn affected by the temperature of the semiconductor surface.

Integrated circuits generally comprise numerous transistors formed nearthe surface of a semiconductor substrate. To form transistors thesemiconductor substrate is doped at selected areas with donor andacceptor impurity atoms to alter the conductivity of the semiconductorand to provide the desired carrier type. The electron (a negativecharge) is the majority carrier in a semiconductor doped with donoratoms. The hole (a positive charge) is the majority carrier in asemiconductor doped with acceptor atoms. The current and voltagecharacteristics of a transistor depend in part on the effective mobilityof the charge carriers.

The physical properties of doped and undoped semiconductor materials aretemperature dependent. The mobility of charge carriers in asemiconductor lattice varies with temperature. The conductivity ofundoped silicon also depends on temperature. The conductivecharacteristics of the transistor are heavily dependent on temperature.The switching speed and performance of the transistors are in turnaffected by the conductive characteristics of the transistor. The outputcharacteristics of an integrated circuit containing millions or evenbillions of transistors can be greatly affected by temperature.

Integrated circuits generally comprise many other kinds of circuitelements whose characteristics are also dependent on the temperature.Integrated circuits are formed of many interconnecting metal linesformed within a multilevel dielectric stack. The physicalcharacteristics of the metal lines and the layers of the dielectricstack also depend on temperature. The temperature dependence of all ofthese components of an integrated circuit makes the outputcharacteristics of the integrated circuit dependent on temperature.

Many factors affect the temperature of an integrated circuit. The veryuse of an integrated circuit will change its temperature. As anintegrated circuit is used, the large amounts of current flowing throughthe many circuit elements cause the temperature of the integratedcircuit to increase. The heat generated by the integrated circuitincreases and decreases as the demand on the integrated circuitincreases and decreases. Thus an integrated circuit can undergo largechanges in temperature based solely on its own performance requirementsfrom moment to moment.

The temperature of the environment in which the integrated circuit isplaced can also have a great effect on the temperature of the integratedcircuit, particularly in very cold climates. For instance, a user of anelectronic device in a very cold location may use the device outside andthen bring the device indoors and cause the device to undergo a largechange in temperature due to the large change in ambient temperature.These large changes in temperature affect the performance of theintegrated circuit.

BRIEF SUMMARY

An integrated circuit is generally operable over a large range oftemperatures. Performance characteristics may vary largely over therange of temperatures in which the circuit can operate. Someapplications may call for particularly steady output characteristics. Insuch applications it may be desirable to maintain the temperature of theintegrated circuit in a selected temperature range while the circuit isoperating. In some applications it may be desirable to maintain thetemperature of the integrated circuit in a selected temperature onlyduring certain portions of operation that call for more steady output.Some applications may call for very small fluctuations in outputcharacteristics. In these applications the temperature range may beselected to be very small according to the output specifications.

An integrated circuit may be heated to maintain the integrated circuitin a selected temperature range. If the integrated circuit is kept inthis smaller temperature range the output characteristics of theintegrated circuit will remain much steadier.

One embodiment is an integrated circuit comprising a semiconductorsubstrate having an active circuit in an active region of thesemiconductor substrate. A temperature sensor is coupled to the activecircuit. The temperature sensor is configured to measure a temperatureof the active circuit. A heating element is coupled to the activecircuit and configured to heat the active circuit. A temperaturecontroller is coupled to the temperature sensor and the heating element.The temperature controller is configured to receive temperature datafrom the temperature sensor and to operate the heating element tomaintain the temperature of the integrated circuit above a selectedtemperature.

In one embodiment the heating element is a thin film heating element.The heating element may be, for example, a TaAl thin film heatingelement.

In one embodiment the heating element is located in a multileveldielectric stack of the integrated circuit. In one embodiment theheating element is located above the active circuit.

In one embodiment the temperature sensor is a bandgap temperaturesensor. In one embodiment the bandgap temperature sensor is in theactive region.

One embodiment is a method comprising measuring a temperature of an ofan integrated circuit, sending temperature data to a temperaturecontroller in the integrated circuit, and activating a heating elementto heat the integrated circuit above a selected temperature. Thetemperature of the integrated circuit is then maintained above aselected minimum temperature.

In one embodiment the temperature of the integrated circuit ismaintained in a selected temperature range.

One embodiment comprises sending a current through a thin film heatingelement to heat the integrated circuit.

One embodiment comprises varying a magnitude of the current according tothe temperature data to maintain the temperature in the selectedtemperature range.

One embodiment is a device comprising a semiconductor die having anactive region. A dielectric stack is located above the semiconductordie. A temperature sensor is in the active region and is configured tomeasure the temperature of the active circuit. A thin film heater islocated in the dielectric stack. A temperature controller is in theactive region and is configured to receive temperature data from thetemperature sensor and to operate the thin film heater according to thetemperature data to maintain the temperature of the active circuit in aselected temperature range.

In one embodiment the temperature controller regulates a current in thethin film heater to generate heat to maintain the temperature of theactive circuit in the selected range.

One embodiment is a portable electronic device comprising a battery, anantenna coupled to the battery, and an integrated circuit coupled to theantenna and the battery. The integrated circuit includes a semiconductordie, a dielectric stack on a surface of the semiconductor die, an activecircuit in the semiconductor die, a temperature sensor in thesemiconductor die, a thin film heater in the dielectric stack, and atemperature controller coupled to the temperature sensor and the thinfilm heater. The temperature sensor is configured to measure atemperature of the active circuit. The thin film heater is configured toheat the active circuit. The temperature controller is configured toreceive a temperature signal from the temperature sensor and to controlthe thin film heater to maintain the temperature of the active circuitin a selected temperature range.

In one embodiment the active circuit is on a first semiconductor die andthe heating element is located on a second semiconductor die coupled tothe first semiconductor die.

In one embodiment the temperature sensor is located on the secondsemiconductor die. Alternatively, the temperature sensor may be locatedon the first semiconductor die.

In one embodiment the temperature controller is located on the secondsemiconductor die. Alternatively the temperature controller may belocated on the first semiconductor die.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a curve of an output parameter of an integrated circuit vs.the temperature of the integrated circuit.

FIG. 2 is a block diagram of a system for maintaining a stabletemperature of an active circuit according to one embodiment.

FIG. 3 is a block diagram of a portable electronic device formaintaining a stable temperature of an active circuit according to oneembodiment.

FIG. 4 is a block diagram of a system for maintaining a stabletemperature of an active circuit according to one embodiment.

FIG. 5 is a block diagram of a system for maintaining a stabletemperature of an active circuit according to one embodiment.

FIGS. 6-10 illustrate successive process steps for manufacturing anintegrated circuit according to one embodiment.

FIG. 11 illustrates an integrated circuit according to one embodiment.

FIG. 12 illustrates an integrated circuit according to one embodiment.

FIG. 13 is a layout for a thin film heating element according to oneembodiment.

FIG. 14 plots the temperature of a thin film heating element vs. thecurrent in the heating element.

FIG. 15 is a block diagram of a system for heating an integrated circuitaccording to one embodiment.

FIG. 16 is a block diagram of a system for heating an integrated circuitaccording to one embodiment.

FIG. 17 is a block diagram of a system for heating an integrated circuitaccording to one embodiment.

FIG. 18 illustrates a two-die configuration of a system for heating anintegrated circuit according to one embodiment.

FIG. 19 illustrates a two-die configuration of a system for heating anintegrated circuit according to one embodiment.

FIG. 20 illustrates a two-die configuration of a system for heating anintegrated circuit according to one embodiment.

FIG. 21 illustrates a two-die configuration of a system for heating anintegrated circuit according to one embodiment.

FIG. 22 illustrates an integrated circuit according to one embodiment.

FIG. 23 illustrates steps of a method for controlling the temperature ofan integrated circuit according to one embodiment.

FIG. 24 illustrates steps of a method for controlling the temperature ofan integrated circuit according to one embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a curve of an output characteristic of an active circuit ofan integrated circuit vs. the temperature of the active circuit. Theoutput characteristic is for example an output voltage, an outputcurrent, switching speed, a signal strength at a given frequency or anyother characteristic that may vary with temperature in any manner. Thecurve of FIG. 1 illustrates the effect a large range in temperature canhave on the output of an integrated circuit. Here the characteristicvaries inversely with temperature. Over the entire range of temperaturethere is a large change ΔP1 in the output characteristic. This can beproblematic for systems which call for steady or even tightly controlledoutput characteristics from an integrated circuit. However, over thesmaller range of temperatures between 100° C. and 110° C. there is amuch smaller change ΔP2 in the output characteristic. Thus maintainingthe temperature of the active circuit in a relatively small range willallow for more steady output characteristics.

FIG. 2 illustrates a block diagram of a system 25 according to oneembodiment. A temperature sensor 32 is positioned adjacent to an activecircuit 30 of an integrated circuit. For the purpose of this disclosureand in the claims, “adjacent” means “near”, “close to”, or otherwise inphysical proximity sufficient to perform a given function relating toanother component or structure. The temperature sensor 32 is positionedsuch that the temperature of the active circuit 30 can either beascertained directly or extrapolated from a parameter of the temperaturesensor 32. The temperature sensor 32 is coupled to a temperaturecontroller 34. The temperature sensor 32 communicates a temperaturesignal to the temperature controller 34. The temperature signal isrepresentative of the temperature of the active circuit 30. Thetemperature controller 34 is coupled to a heating element 36. Theheating element 36 generates and provides heat to the active circuit 30.The temperature controller 34 controls the function of the heatingelement 36. The temperature controller 34 activates the heating element36 and controls a level of heat generated by the heating element 36 tomaintain a temperature of the active circuit 30 above a selected minimumtemperature. The temperature controller controls the heating element togenerate more heat at times when the temperature of the active circuitrisks falling below the minimum temperature of the temperature range andgenerates less heat at times when the temperature is at less risk offalling below the selected minimum temperature.

Some systems and applications are such that they call for particularlysteady output characteristics during operation of the active circuit 30or during certain portions of operation of the active circuit 30. Inthese cases the temperature controller 34 controls the temperature ofthe active circuit 30 above a selected minimum temperature and below aselected maximum temperature. In other words, the temperature controllerensures that the temperature of the active circuit 30 remains in aparticular temperature range. The size of the temperature rangedetermines the potential variation in output characteristics. Thesmaller the range of temperatures over which the active circuit 30operates, the steadier the output characteristics will be. Applicationsthat call for very steady output characteristics can be operated in aparticularly tight temperature range. In these applications thetemperature controller 34 controls the heating element 36 to generatemore heat when the active circuit 30 risks falling below the minimumtemperature of the selected temperature range or to generate less heatwhen the active circuit 30 risks surpassing the maximum temperature ofthe selected temperature range.

As discussed above, the active circuit 30 is itself a source of heatthat alters the temperature of the active circuit 30. As demand on theactive circuit 30 increases, so does the heat generated by the activecircuit 30, and the temperature of the active circuit 30 rises. Theamount of heat generated by the active circuit 30 decreases as demand onthe active circuit 30 decreases. This increase and decrease in heatgenerated by the active circuit 30 due to the variable level offunctioning of the active circuit 30 also affects the temperature of theactive circuit 30.

In one embodiment the minimum temperature of the selected temperaturerange is chosen to be a temperature at which the active circuit 30 wouldby itself operate when in a state of relatively high demand. The heatingelement 36 is utilized to maintain the temperature of the active circuit30 above this minimum temperature. In this way the temperature of theactive circuit will not surpass the temperature range simply byoperating in a common state of high demand. Of course in otherembodiments the temperature range may be selected to coincide with atemperature range in which the active circuit would by itself operatewhen in a state of low or moderate demand.

In one embodiment the temperature controller 34 takes into account themomentary demand on the active circuit 30 when determining the desiredheat output of the heating element 36. When the demand on the activecircuit 30 is low, the temperature controller 34 controls the heatingelement 36 to generate more heat to maintain the temperature of theactive circuit above the minimum temperature. When demand on the activecircuit 30 is high, the temperature controller 34 controls the heatingelement 36 to generate less heat in order to maintain the temperature ofthe active circuit 30 below the maximum temperature of the selectedtemperature range.

The temperature sensor 32, the temperature controller 34, and theheating element 36 can thus be utilized to maintain the temperature ofthe active circuit 30 in a selected temperature range. The temperaturesensor 32 continually or periodically measures the temperature of theactive circuit 30 and communicates a temperature signal to thetemperature controller 34. The temperature controller 34 calculates alevel of heat to output from the heating element 36 to maintain thedesired temperature of the active circuit 30. The temperature controller34 then controls the level of heat output of the integrated circuit. Thetemperature controller 34 adjusts the heat output of the heating element36 based on the temperature signal from the temperature sensor 32.

In one embodiment, the temperature controller 34 is coupled to theactive circuit 30 so as to receive data from the active circuit 30regarding a level of function of the active circuit 30. The temperaturecontroller 34 can then use the temperature signal and the data regardingthe level of function of the active circuit 30 to calculate the heat tooutput from the heating element 36 to maintain the temperature of theactive circuit 30 in the desired temperature range.

In one embodiment the temperature controller 34 makes calculations byreferencing a database stored in a physical memory 38 coupled to thetemperature controller 34. The memory 38 stores data relating to thelevel of function of the active circuit 30, the heat output of theheating element 36, and the temperature of the active circuit 30. Thetemperature controller 34 is configured to write data to the memory 38based on new temperature measurements. The memory 38 is in the form ofEEPROM, Flash memory, magnetic hard drive, or any other suitable memoryfrom which the temperature controller 34 or other circuit components mayread and/or write data.

FIG. 3 shows a block diagram of a wireless electronic device 40according to one embodiment. The wireless electronic device 40 is forexample a cell phone, a PDA, an MP3 player, a laptop, or other wirelessdevice.

The wireless electronic device shows an active circuit 30, a temperaturesensor 32, a temperature controller 34, a memory 38, and a heatingelement 36 as shown in FIG. 2. FIG. 3 additionally illustrates anantenna circuit 42 and a display coupled to the active circuit 30. Abattery is coupled to the active circuit 30 and the temperaturecontroller 34. In practice there are many more components in suchdevices than are illustrated in FIG. 3.

In extremely cold climates many electronic communication devicesfunction poorly or not at all. In some cases the display fails tofunction, active circuit 30 fails to turn on, or active circuit 30performs very poorly. Analog circuitry can be particularly affected byextremes in temperature.

In one embodiment, upon turning on the wireless electronic device 40,the temperature controller 34 activates the heating element 36 to beginheating the active circuit 30 so that it may turn on or functionproperly. In alternative embodiments the heating element 36 is utilizedto heat display circuitry 44, antenna circuit 42, signal processingcircuitry, I/O circuitry, processing circuitry, control circuitry,memory circuitry 38, or any other circuitry in the wireless electronicdevice 40. In other words the active circuit 30 may take the form of anyof the circuitry mentioned above or any other circuitry that benefitsfrom use of the heating element 36 in any way. The blocks used in FIGS.2 and 3 are merely exemplary and may be combined or utilized in anysuitable configuration.

Once the desired circuitry has been sufficiently heated, the temperaturesensor 32, the temperature controller 34, and the heating element 36 canbe utilized to maintain the temperature of the active circuit 30 asdescribed in relation to FIG. 2.

While the embodiment illustrated in FIG. 3 is a wireless electronicdevice 40, other embodiments include any kind of electronic device,wireless or otherwise, that may benefit from the heating processdescribed in relation to FIG. 2.

FIG. 4 illustrates a block diagram of a system 25 according to oneembodiment. A band gap temperature sensor 32 monitors the temperature ofthe active circuit 30. The band gap of a semiconductor substrate 48varies slightly with temperature. Thus the temperature of thesemiconductor substrate 48 can be measured by measuring (directly orindirectly) the band gap. The band gap temperature sensor 32 ispositioned adjacent to the active circuit 30 so as to be able togenerate a temperature signal that is representative of the temperatureof the active circuit 30. In one embodiment the band gap temperaturesensor 32 is a Brokaw band gap temperature sensor 32, but any suitableband gap sensor may be used.

The band gap temperature sensor 32 is coupled to a microcontroller 34,acting as temperature controller. The microcontroller 34 receives atemperature signal from the band gap temperature sensor 32. Thetemperature signal is representative of the temperature of the activecircuit 30. The microcontroller 34 is coupled to a thin film heatingelement 36. The thin film heating element 36 generates heat when anelectric current is sent through it. The larger the current in the thinfilm heating element 36, the larger the heat output from the thin filmheating element 36. The thin film heating element 36 is a thin film ofany suitable material that heats up as current goes through it. Heatenergy from the thin film heating element 36 diffuses and heats up theactive circuit 30. The active circuit 30 and the thin film heatingelement 36 are arranged so that the thin film heating element 36 mayaffect the temperature of the active circuit 30. In one embodiment thethin film heating element 36 is a TaAl thin film heating element 36.

The microcontroller 34 is configured to control the amount of current inthe thin film heating element 36. By controlling the amount of currentin the thin film heating element 36 the microcontroller 34 controls theheat output of the thin film heating element 36. As the microcontroller34 receives temperature signals from the band gap temperature sensor 32,the microcontroller 34 varies the current in the thin film heatingelement 36 to control the heat output from the thin film heating element36 and to maintain the temperature of the active circuit 30 in aselected range as described in relation to FIG. 2.

FIG. 5 illustrates one embodiment in which the temperature sensor 32 isimplemented as a resistive temperature sensor 32. The resistivetemperature sensor 32 is implemented in a material whose resistancechanges with temperature. The temperature of the resistive temperaturesensor 32 is determined by a measurement of its resistance. Theresistive temperature sensor 32 is positioned relative to the activecircuit 30 such that a measurement of the temperature of the resistivetemperature sensor 32 is indicative of the temperature of the activecircuit 30. In one embodiment, the temperature of the active circuit 30is extrapolated from a measurement of the resistance of the resistivetemperature sensor 32. In one embodiment the resistive temperaturesensor 32 is a thin film resistor whose resistance varies with thetemperature. In one embodiment the resistive temperature sensor 32 is athin film resistor made from CrSi.

FIGS. 6-10 illustrate a simplified process for manufacturing anintegrated circuit according to one embodiment. In FIG. 6 an activecircuit 30, a band gap temperature sensor 32, and a temperaturecontroller 34 have been formed in a semiconductor substrate 48. As shownin FIG. 6, the band gap temperature sensor 32 is located in thesemiconductor substrate 48 near the active circuit 30. In one embodimentthe band gap temperature sensor 32 and/or the temperature controller 34are formed as part of the active circuit 30. Many other configurationsof the active circuit 30, the temperature controller 34, and the bandgap temperature sensor 32 are also possible as will be readily apparentto those of skill in the art in light of the present disclosure.

In FIG. 7 a pre-metal dielectric layer 50, for example a silicon nitrideor silicon oxide layer, is deposited on the substrate 48. Contacts 52 tothe active device and the temperature controller 34 are etched andfilled. In FIG. 8 metal lines 54 are formed overlaying the contacts 52.In FIG. 9 the TaAl thin film heating element 36 is formed between twometal lines 54. The thin film heating element 36 is connected by one ofthe metal lines 54 to the temperature controller 34. The temperaturecontroller 34 controls the amount of current that passes through thethin film heating element 36.

In FIG. 10 a second dielectric layer 56, for example a silicon oxide ora silicon nitride layer, has been deposited over the thin film heatingelement 36 and the metal lines 54. Second contacts 57 are made in thesecond dielectric layer 56 and filled. Second metal lines 58 are madeoverlying the second dielectric layer 56 and second contacts 57. Theintegrated circuit is then passivated and packaged (not shown). It isunderstood by those of skill in the art that many process steps andstructures have not been illustrated in FIGS. 6-10 for the sake ofsimplicity. Such steps and structures are known by those of skill in theart and can now readily be integrated with embodiments in light of thepresent disclosure. All such structures and steps fall within the scopeof the present disclosure.

FIG. 11 illustrates an integrated circuit containing a thin film heatingelement 36 according to an alternate embodiment. The process steps arenot illustrated and the structures are simplified. On a semiconductorsubstrate 48, a pre-metal dielectric layer 50 is deposited. The heightof the pre-metal dielectric layer 50 is for example 8 kÅ (kiloAngstroms). First metal conducting lines 54 are deposited on thepre-metal dielectric 50. A first inter-level dielectric layer 59 isdeposited on the pre-metal dielectric 50. The height of the firstinter-level dielectric layer 59 is for example 5 kÅ. The TaAl thin filmheating element 36 is deposited on the first inter-level dielectriclayer 59. A second inter-level dielectric layer 60 is deposited on thefirst inter-level dielectric layer 59 and the thin film heating element36. The height of the second inter-level dielectric layer 60 is forexample 5 kÅ. A first contact via 62 is made through the first andsecond inter-level dielectric layers 59, 60 to one of the first metallines 54. A second contact via 64 is made through the second inter-leveldielectric layer 60 to the thin film heating element 36. The firstcontact via 62 and the second contact via 64 are filled with conductivematerial. A second metal track 66 is formed on the second inter-leveldielectric layer 60. A third inter-level dielectric layer 68 is formedon the second inter-level dielectric layer 60 and the second metal track66. The height of the third inter-level dielectric is for example 5 kÅ.A third contact via 70 is made through the third inter-level dielectriclayer 68 to the second metal track 66 and filled. A third metal track 72is deposited on the third inter-level dielectric layer 68. A passivationlayer 74 is formed on the third inter-level dielectric layer 68 andthird metal track 72. The height of the passivation layer 74 is forexample 10 kÅ. A portion of the passivation layer 74 is etched to exposea portion of the third metal track 72. A conductive barrier layer 76 isdeposited over the exposed portion of the third metal track 72 and asolder ball 78 is placed on the conductive barrier 76.

An example of relative temperatures in the integrated circuit accordingto one embodiment will now be described. In one embodiment it isdesirable to keep the temperature of the active circuit 30 between 50°C. and 55° C. in order to stabilize output parameters of the activecircuit 30. The active circuit 30 is not shown in FIG. 11, but islocated in the substrate 48 below the thin film heating element 36. Inother embodiments the active circuit 30 is located in other positionsrelative to the heating element 36.

In this example the ambient temperature outside of the integratedcircuit is 20° C. The microcontroller 34 (also not shown in FIG. 11, butlocated within the silicon substrate 48) sends a current through thethin film heating element 36 such that the temperature of the thin filmheating element 36 is 200° C. Heat from the thin film heating element 36diffuses throughout the integrated circuit. Areas further from theheating element 36 will be heated less than areas closer to the heatingelement 36. The temperature at the junction of the pre-metal dielectriclayer 50 and the first inter-level dielectric layer 59 is for example100° C. The temperature of the active circuit in the silicon substrate48 is for example 50° C. The temperature sensor 32 detects thistemperature and sends a temperature signal to the temperature controller34 which calculates an amount of current to send through the heatingelement 36 according to the temperature data. In this way thetemperature of the active circuit 30 can be maintained between 50° C.and 55° C. In practice the relative temperatures of the heating element36, the dielectric layers, and the active circuit 30 may be verydifferent from this example and will depend on the structure of theintegrated circuit, the relative placement of the integrated circuit,the materials used in the integrated circuit, and so forth. All of theseparameters can be taken into account when manufacturing an integratedcircuit and tests can also be run in order to determine how thetemperature of the active circuit 30 will respond to the temperature ofthe heating element 36.

FIG. 12 illustrates an integrated circuit containing the thin filmheating element 36 according to one embodiment. Features similar tothose of FIG. 11 receive the same reference numbers. On a semiconductorsubstrate 48, a pre-metal dielectric layer 50 is deposited. The heightof the pre-metal dielectric layer 50 is for example 8 kÅ. First metalconducting lines 54 are deposited on the pre-metal dielectric layer 50.A first inter-level dielectric layer 59 is deposited on the pre-metaldielectric layer 50. The height of the first inter-level dielectriclayer 59 is for example 5 kÅ. A second inter-level dielectric layer 60is deposited on the first inter-level dielectric layer 59. The height ofthe second inter-level dielectric layer 60 is for example 5 kÅ. A firstcontact via 62 is made through the first and second inter-leveldielectric layers 59, 60 to one of the first metal lines 54 and filled.A second metal track 66 is formed on the second inter-level dielectriclayer 60. A third inter-level dielectric 68 is formed on the secondinter-level dielectric layer 60 and the second metal track 66. Theheight of the third inter-level dielectric layer 68 is for example 5 kÅ.A second contact via 70 is made through the third inter-level dielectriclayer 68 to a first portion of the second metal track 66 and filled. Athird contact via 64 is made through the third inter-level dielectriclayer 68 to the second metal track 66 and filled. The TaAl thin filmheating element 36 is formed in the third inter-level dielectric layer68 with a portion of the thin film heating element 36 contacting theexposed second portion of the second metal track 66 to make anelectrical connection between the thin film heating element 36 and thesecond metal track 66. A third metal track 72 is deposited on the thirdinter-level dielectric layer 68. A passivation layer 74 is deposited onthe third inter-level dielectric layer 68 and third metal track 72. Theheight of the third inter-level dielectric layer 68 is for example 10kÅ. A portion of the passivation layer 74 is etched to expose a portionof the third metal track 72. A conductive barrier layer 76 is depositedover the exposed portion of the third metal track 72 and a solder ball78 is placed on the conductive barrier 76.

FIG. 13 is top view of the layout of a TaAl thin film heating element 36according to one embodiment. Because the dimensions of features on anintegrated circuit are typically very small, it is often advantageous toimplement a snake-like configuration for a resistor in an integratedcircuit. The many connected segments increase the length of the resistorand thus allow the resistor to achieve a desired level of resistance. Inone embodiment the thickness of the TaAl thin film heating element 36 isabout 200 nm. In one embodiment the sheet resistance of the TaAl thinfilm heating element 36 is 10-100Ω/sq. In one embodiment the resistanceof the TaAl thin film heating element 36 is 120Ω. The resistance of theTaAl thin film heating element 36 can be more or less than thisdepending on the needs of any particular device. Of course, any othersuitable material may be used in place of TaAl to implement the thinfilm heating element 36.

FIG. 14 is a plot of the simulated temperature of the thin film heatingelement 36 as a function of the current through the device. As describedabove, in one embodiment the temperature controller 34 varies themagnitude of the current in the thin film heating element 36 in order togenerate more or less heat as needed to control the temperature of theactive circuit 30. In one embodiment the temperature of the thin filmheating element 36 is about 200° C. when the current in the thin filmheating element 36 is about 30 mA. The temperature of the active circuit30 will depend in part on the temperature of the heating element 36 andthe position of the heating element 36 in relation to the active circuit30.

In one embodiment the active circuit 30 is implemented in a firstsemiconductor die 80 and the heating element 36 is implemented in asecond semiconductor die 82. In extremely cold temperatures someintegrated circuits function poorly or are unable to turn on at all. Inthis configuration the second semiconductor die 82 can act as anignition die for the active circuit 30 on the first semiconductor die80. In cold temperatures, the heating element 36 on the secondsemiconductor die 82 is first activated to heat the active circuit 30 toa desired temperature. When the active circuit 30 is sufficiently heatedit may then turn on and function properly. The heating element 36 isthen used to perform the function of heating the active circuit 30 tomaintain the temperature of the active circuit 30 in a selectedtemperature range.

FIG. 15 is a block diagram of an embodiment in which the active circuit30 is located on a first semiconductor die 80 and the heating element 36is located on a second semiconductor die 82. In the embodiment of FIG.15 the heating element 36, the temperature sensor 32, and thetemperature controller 34 are all located on the second semiconductordie 82. The second semiconductor die 82 is coupled to the firstsemiconductor die 80 in such a way that heat from the heating element 36can diffuse and heat the active circuit 30 in the first semiconductordie 80. The temperature sensor 32 is positioned relative to the heatingelement 36 and the first semiconductor die 80 such that the temperatureof the active circuit 30 can be extrapolated from the temperaturemeasured by the temperature sensor 32. In one embodiment the temperaturesensor 32 is much closer to the heating element 36 than to the activecircuit 30. In this case the temperature of the active circuit 30 may bemuch less than the temperature measured by the temperature sensor 32. Asdescribed above in relation to the exemplary temperature data presentedin relation to FIG. 11, by measurement and calculation the relationbetween the temperature at the position of the temperature sensor 32 inthe second semiconductor die 82 and the active circuit 30 in the firstsemiconductor die 80 can be known. This relation can be stored in memory38 or otherwise made available to the temperature controller 34. Thetemperature controller 34 can then accurately control the heatingelement 36 to heat the active circuit 30 to maintain the temperature ofthe active circuit 30 in the selected temperature range.

In the embodiment of FIG. 16 the temperature sensor 32 is located on thefirst semiconductor die 80 with the active circuit 30. The heatingelement 36 and temperature controller 34 are located on the secondsemiconductor die 82. The temperature sensor 32 is electrically coupledto the temperature controller 34. The temperature controller 34 islocated on the second semiconductor die 82 and controls the temperatureof the heating element 36 according to temperature signals received fromthe temperature sensor 32.

In the embodiment of FIG. 17 the temperature sensor 32, the temperaturecontroller 34, and the active circuit 30 are all located on the firstsemiconductor die 80. The heating element 36 is located on the secondsemiconductor die 82. The heating element 36 is electrically connectedto the temperature controller 34 so that the temperature controller 34can control an operation of the heating element 36.

FIGS. 18-21 illustrate exemplary configurations in which the secondsemiconductor die 82 is coupled to the first semiconductor die 80. Inone embodiment the first and the second semiconductor dies 80, 82 areseparately passivated after manufacture and then attached to each other.The two dies are then attached to a circuit board 84.

In FIG. 18 the second semiconductor die 82 is attached to the firstsemiconductor die 80 by an adhesive layer 83. The second semiconductordie 82 is electrically connected to bonding pads (not shown) on thecircuit board 84 by means of bonding wires 86. The first semiconductordie 80 is coupled to the circuit board 84 by solder balls 78.

In FIG. 19 the second semiconductor die 82 is coupled to the firstsemiconductor die 80 by solder bumps 88. The solder bumps 88 can providean electrical connection between the first semiconductor die 80 and thesecond semiconductor die 82 so that components on the firstsemiconductor die 80 can communicate with components on the secondsemiconductor die 82. The solder bumps 88 can also function to improveheat transfer from the second semiconductor die 82 to the firstsemiconductor die 80. The second semiconductor die 82 is coupled to thecircuit board 84 by solder balls 78.

In FIG. 20 the second semiconductor die 82 is coupled to the firstsemiconductor die 80 by solder bumps 88. The solder bumps 88 can providean electrical connection between the first semiconductor die 80 and thesecond semiconductor die 82 so that components on the firstsemiconductor die 80 can communicate with components on the secondsemiconductor die 82. The first semiconductor die 80 is coupled to thecircuit board 84 by an adhesive layer 83. The first semiconductor die 80is electrically connected to the circuit board 84 by bonding wires 86.

In FIG. 21 the second semiconductor die 82 is coupled to the firstsemiconductor die 80 by solder bumps 88. The solder bumps 88 can providean electrical connection between the first semiconductor die 80 and thesecond semiconductor die 82 so that components on the firstsemiconductor die 80 can communicate with components on the secondsemiconductor die 82. The solder bumps 88 can also function to improveheat transfer from the second semiconductor die 82 to the firstsemiconductor die 80. The second semiconductor die 82 is coupled to thecircuit board 84 by solder balls 78. The second semiconductor die 82 iselectrically connected to the circuit board 84 by bonding wires 86.

In one embodiment the temperature sensor 32 is a resistive temperaturesensor 32 located on the second semiconductor die 82. The resistance ofthe resistive temperature sensor 32 is temperature dependent. Theresistance of the resistive temperature sensor 32 is representative ofthe temperature of the resistive temperature sensor 32. The temperatureof the resistive temperature sensor 32 is representative of thetemperature of the active circuit 30 according to a relationship whichcan be calculated and measured as described above.

In one embodiment the resistive temperature sensor 32 is a thin filmresistor made of CrSi. In one embodiment the resistance of the CrSiresistive temperature sensor 32 varies by 4000 ppm/C. In other words theresistance changes by 0.004% for a change in temperature of 1° C. Byknowing the resistance at a given temperature (which can be obtained byprior measurement), the temperature of the resistive temperature sensor32 can be calculated based on its resistance. In one embodiment thevoltage across the resistive temperature sensor is indicative of thetemperature of the active circuit 30.

FIG. 22 illustrates an embodiment in which the heating element 36 andthe resistive temperature sensor 32 are implemented on the secondsemiconductor die 82. The lower layers of the dielectric stack and thelower metal layers are not illustrated. A first metal track 92, a secondmetal track 93, and a third metal track 94 are shown on a firstdielectric layer 90 of Si₃N₄. A CrSi resistive temperature sensor 32 isthen formed on the second metal track 93. A second dielectric layer 91of Si₃N₄ is then deposited on the first dielectric layer 90 of Si₃N₄,the resistive temperature sensor 32, the first metal track 92 and thethird metal track 94. The second dielectric layer 91 is etched to exposethe first metal track 92. A TaAl thin film heating element 36 isdeposited on the second dielectric layer 91 and the exposed portion ofthe first metal track 92. A third dielectric layer 95 of SiO₂ is thendeposited over the heating element 36 and the second dielectric layer91. The third dielectric layer 95 is then etched to expose a portion ofthe heating element 36 and the third metal track 94. Plugs fill theetched portions and solder bumps 88 are attached to the plugs.

FIG. 23 illustrates a method according to one embodiment. At 100, thetemperature sensor 32 measures the temperature of the active circuit 30.At 102 the temperature controller 34 receives a temperature signal fromthe temperature sensor 32 and computes the output of the heating element36 that should be applied. At 104 the temperature controller 34 controlsheating element 36 to output the desired heat to maintain thetemperature of the active circuit 30 above a selected minimumtemperature or in a selected temperature range. Steps 100-104 arerepeated throughout any period during which it is desired to maintainthe temperature of the active circuit 30 in the selected temperaturerange.

FIG. 24 illustrates a method according to one embodiment. At 200 abandgap temperature sensor 32 monitors the temperature of the activecircuit 30. At 202 the bandgap temperature sensor 32 communicates to thetemperature controller 34 a temperature signal representative of thetemperature of the active circuit 30. At 204 the temperature controller34 calculates an amount of current to apply to a thin film heatingelement 36 according to the temperature signal in order to maintain thetemperature of the active circuit 30 in a selected temperature range. At206 the temperature controller 34 sends a current through a thin filmheating element 36 to generate heat to heat the active circuit 30 tomaintain the temperature of the active circuit 30 in the selectedtemperature range.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A device comprising: a semiconductor substrate having an activeregion; an active circuit in the active region; a temperature sensoradjacent to the active circuit, the temperature sensor being configuredto measure a temperature of the active circuit; a heating elementadjacent to the active circuit and configured to heat the activecircuit; and a temperature controller coupled to the temperature sensorand the heating element, the temperature controller being configured toreceive temperature data from the temperature sensor and to cause theheating element to maintain the temperature of the integrated circuitabove a threshold temperature.
 2. The device of claim 1 wherein theheating element is a thin film heating element.
 3. The device of claim 2wherein the heating element is TaAl.
 4. The device of claim 2 comprisinga multilayer dielectric stack on a surface of the semiconductorsubstrate, the thin film heating element being located in the multilayerdielectric stack.
 5. The device of claim 2 wherein the heating elementis located above the active circuit.
 6. The device of claim 2 whereinthe heating element is on a surface of the semiconductor substrate,adjacent to the active circuit.
 7. The device of claim 1 wherein thetemperature sensor is a bandgap temperature sensor.
 8. A methodcomprising: generating a temperature signal representative of atemperature of an active circuit in an integrated circuit; receiving thetemperature signal in a temperature controller; heating the activecircuit above a minimum threshold temperature, the heating includingoperating a heating element coupled to the integrated circuit to heatthe integrated circuit above the minimum threshold temperature; andmaintaining the temperature of the active circuit above the minimumthreshold temperature, the maintaining including varying a heat outputof the heating element according to the temperature signal to maintainthe temperature of the integrated circuit above the minimum thresholdtemperature.
 9. The method of claim 8 wherein maintaining includesvarying the heat output of the heating element to maintain thetemperature of the integrated circuit above the minimum thresholdtemperature and below a maximum threshold temperature.
 10. The method ofclaim 9 wherein the heating element is a thin film heating elementlocated in a dielectric stack of the integrated circuit.
 11. The methodof claim 10 wherein the heating includes sending a current through thethin film heating element to heat the integrated circuit.
 12. The methodof claim 11 wherein the maintaining includes varying a magnitude of thecurrent according to the temperature data to maintain the temperatureabove the minimum threshold temperature and below the maximum thresholdtemperature.
 13. A device comprising: a first semiconductor die; anactive circuit in the first semiconductor die; a temperature sensorconfigured to measure a temperature of the active circuit; a secondsemiconductor die coupled to the first semiconductor die; a heatingelement on the second semiconductor die, the heating element configuredto heat the active circuit of the first semiconductor die; and atemperature controller configured to receive temperature data from thetemperature sensor, to operate the heater according to the temperaturedata, and to cause the heater to heat the active circuit to maintain thetemperature of the active circuit above a minimum threshold temperatureand below a maximum threshold temperature.
 14. The device of claim 13wherein the temperature controller is in the second semiconductor die.15. The device of claim 13 wherein the temperature controller is in thefirst semiconductor die.
 16. The device of claim 13 wherein thetemperature controller regulates a current in the thin film heater togenerate heat to maintain the temperature of the active circuit abovethe minimum threshold temperature and below the maximum thresholdtemperature.
 17. The device of claim 13 wherein the temperature sensoris on the second semiconductor die;
 18. The device of claim 17 whereinthe temperature sensor is a thin film resistor whose resistance varieswith temperature.
 19. A wireless device comprising: a battery; anantenna coupled to the battery; and an integrated circuit packagecoupled to the antenna and the battery, the integrated circuit packageincluding: a semiconductor die; an active circuit in the semiconductordie; a temperature sensor in the semiconductor die, the temperaturesensor configured to measure a temperature of the active circuit; adielectric stack on a surface of the semiconductor die; a thin filmheater in the dielectric stack, the thin film heater configured to heatthe active circuit; and a temperature controller configured to receivetemperature data from the temperature sensor and to cause the thin filmheater to heat the active circuit to maintain the temperature of theactive circuit above a minimum threshold temperature and below a maximumthreshold temperature.
 20. The wireless device of claim 19 comprising amemory coupled to the temperature controller.
 21. The wireless device ofclaim 20 wherein temperature controller reads data from the memory tocalculate the heat to output from the thin film heater to maintain thetemperature of the active circuit above the minimum thresholdtemperature and below the maximum threshold temperature.